RainScreen Design

Rainscreens are time tested wall systems that have been in use for over a century. The rainscreen design is generally defined by the separation of cladding from a structural wall in an effort to manage moisture and energy transfer through a wall assembly Advances in technology over the past few decades have optimized the design from a simple two stage weather-tightening system to the inclusion of a ventilated airspace and high-performance products that make up the entire assembly. A rainscreen typically consists of the following components:

• Exterior cladding (with or without open joints)

• Ventilation and drainage cavity

• Insulation

• Air barrier

• Structure of building

The Elements

The key to a properly designed rain screen wall system is to control and manage nature’s elements, which are:

• Water infiltration

• Ultra-violet radiation

• Negative wind pressures

• Heat transfer into and out of the building

• Air infiltration

• Vapor transmission

A rainscreen system is designed to allow the remaining components of the wall assembly to effectively manage this moisture, along with the other elements, to provide a long-lasting, energy efficient, high-performance wall system.

System Components

Metal Cladding –As the first line of defense, a high quality wall panel cladding system is essential for a number of reasons in a rainscreen assembly. In addition to protecting the wall assembly against the majority of rain, it also protects against negative wind pressures and ultra-violet radiation.

When considering more economical options, the wall panel system should be comprised of a Galvalume®or G-90 galvanized coated steel, or high-quality aluminum with a fluorocarbon paint finish, such as Kynar®. For a more premium design, corrosion resistant metal materials such as zinc, copper or stainless steel provide for a highly attractive, longer-lasting cladding system. With the rainscreen design, the metal wall panel system may contain open joints that are not as installer sensitive as some face-sealed wall panels, which drastically reduces maintenance throughout the life of the system.

Ventilation/Drainage Cavity and Framing System –The ventilation cavity is a crucial component of the rainscreen wall assembly. It must be incorporated into the system via the framing system of the metal wall cladding. This framing system should be designed to not only support the wall panels, but also provide a means to promote ventilation behind the cladding. This cavity promotes residual water drainage and air flow behind the wall panel, which helps to dry out any water that gets through the exterior cladding, as well as ventilates moisture-laden air that may be drawn into the cavity.

The ventilation cavity can be achieved a number of ways, such as a vented hat channel system, along with vented metal trim components at the bottom and top of the wall panel assembly. The metal wall panel system anchors into the hat channel, which is fastened through the continuous layer of insulation and into the structural assembly of the wall. These fasteners are the only metal components that penetrate through the entire wall assembly, which greatly reduces thermal bridging. According to Building Science Corporation1, failure to break thermal bridging of metal stud components through the insulation of a wall system can reduce their effective R-value by 50-80%. The vented hat channel method eliminates this thermal bridge, allowing the effective R-value of the continuous layer of insulation to remain nearly unchanged.

Insulation –The insulation in a wall assembly reduces the heat flow into and out of the building. For the most effective and energy efficient design, insulation should be applied in a continuous layer on the exterior side of the air barrier.

The insulation used in a rainscreen should be comprised of a material that can handle small amounts of moisture and has the ability to dry without degrading the insulation material or reducing its effective R-value. The ideal types of insulation for rainscreen design include rock wool or extruded polystyrene.

However, if extruded polystyrene is used, it must meet the building code requirements of a fire-rated assembly using foam plastic insulation. Some manufacturers do have these fire-rated options available.

Air Barrier–The air barrier, which is actually the air and water barrier, is the most critical component of the rainscreen assembly. It is the last and most critical line of defense against water infiltration. Any residual amounts of water that bypass the external cladding will ultimately be stopped by this air barrier. Any water that contacts this surface will either drain out of the bottom of the wall assembly, or will evaporate out of the wall.

The air barrier, by definition, stops the flow of air through a wall assembly. This air flow can come from pressure differentials between the exterior and interior climate caused by wind, mechanical and stack effect pressures.Without the air barrier, moving air can also migrate large quantities of vapor through the wall. If this vapor comes in contact with a cool surface, it can condense within the wall assembly, potentially leading to more severe issues.

It is important to understand that air control and vapor control are two separate entities. While the movement of the air can be stopped by the air barrier, vapor transmission can either be stopped or allowed to pass through the air barrier, depending on the material used.

In some climates and building design, vapor transmission through the air barrier should be allowed to promote the drying of materials and, in other scenarios, the vapor should be stopped completely.Proper design and application of the air barrier system is essential. Attention to detail is absolutely critical to assure air barriers within a wall properly tie into roof system air barriers, windows, penetrations and foundations.

In summary, rainscreens provide an ideal wall that includes cladding, a ventilation cavity, insulation, an air barrier and a structural wall that are integrated in a manner to manage and control moisture and energy.

This design is ideal for new construction and retrofit applications alike. Depending on the building design, geographic location, and the use of the building, rainscreens can be designed to suit the needs of each and every facility.The finished product will provide a beautiful, long-lasting, high-performance wall system, allowing a building owner to focus more on their business instead of the building that protects it.

The Two Types of Rainscreens:

Rainscreen systems come in two distinct types: Drained/Back Ventilated, and Pressure Equalized.

Drained/Back-ventilated Rainscreens:

The current system employed by most system manufacturers is drained and back ventilated. This is a worthy system provided you have reasonable workmanship on the air/water barrier. It involves the assumption that water enters the cavity in limited amounts and is prevented from entering the building by the waterproofing. Additionally, the moisture can easily dry since the cavity is vented, causing the water to simply evaporate.

The system in force in most panel installations does not pressure equalize because of the upward or downward or sideways flow of air in a large cavity. Specifically, these large volumes of cavity space behind the panel system do not allow rapid equalization of pressures (if equalization occurs at all).

Therefore the planned scenario is that at least small amounts of water will enter… but then dry out due to the ventilation that is free flowing and available.

Pressure Equalized Rainscreens:

In contrast, the pressure equalized system differs in that the cavity will be broken up horizontally as well as vertically so that individual (smaller) panel areas can pressure equalize. The amount of venting is then carefully calculated based on the cavity volume.

The benefit is that pressure can build up rapidly in the cavity and push back at the same pressure that it is pushing in the vents/seams and gaps, resulting in less water (or no water) entering the system.

Another advantage is that the pressure presses equally on both sides of the panel resulting in a theoretical Net 0 psf on the panel system itself. Consequently, the building design pressure can be applied to the wall behind the panel system. Although this is potentially true, building codes and safety factors require that you design fastening systems able to withstand the design pressures.

Rain Screen Wall Cladding Systems Testing

The new test methods that were developed, approved and published by the AAMA are known as:

AAMA 508-07, “Voluntary Test Method and Specification for Pressure Equalized Rain Screen Wall Cladding Systems” and AAMA 509-09, “Voluntary Test and Classification Method of Drained and Back Ventilated Rain Screen Wall Cladding Systems.”

similarities:

• Both test methods establish a standard specimen size and configuration, requiring an 8 foot by 8 foot square mock-up with at least one vertical and one horizontal joint between panels

• Both test methods allow for testing with a “generic” air/water barrier (AWB), essentially a clear plastic sheet (i.e. Lexan) which allows for visual observation from the interior side of the specimen;

• Both test methods require purposely designed defects (holes) in the generic AWB, for the purpose of understanding the systems performance when applied over an imperfect air barrier. The notion is that under real world conditions, the air barrier isn’t perfect, and introducing a pressure change across the AWB will serve to create conditions that better emulate what will most likely occur in the field.

• Both tests methods allow for the option of structural testing, when using the actual AWB intended for a given job or system design, but the default is the generic AWB.

AAMA 508-07

his test method is intended for products that are defined by the manufacturer as being “Pressure Equalized Rain Screen Wall Cladding Systems” – panels systems, not to be confused with fenestration products. According to the standard there are four (4)essential design requirements:

1. Water entry through the entire wall shall be prevented;

2. If water vapor diffuses through the interior wall construction (from the inside out) then it shall be vented to the exterior;

3. The actual AWB (when tested) shall be designed to resist the full positive and negative wind load;

4. The system shall be designed such that it doesn’t trap or hold concealed water and that it is able to control rain water.

In terms performance, the AAMA 508 test method pass/fail criteria is…

• Failure is defined as water mist or droplets appearing in excess of 5% of the AWB surface; and/or

• Water running in a continuous stream (streaming) down the AWB surface.

• The assembly is also tested to determine if it behaves as a pressure equalized system, which for this test method is based on the lag time between the cyclic wind pressure and the cavity pressure. The lag time shall not exceed 0.08 seconds.

• Finally, the maximum differential between the cyclic wind pressure and the cavity pressure shall not exceed 50% of the maximum pressure when tested at 25 psf for 100 cycles.

AAMA 509-09

This test method is intended for products that are defined by the manufacturer as being “Drained and Back Ventilated Rain Screen Wall Cladding Systems”. In this case it is understood that water will most likely reach the AWB; and that is acceptable. The question is how much water and whether the system is capable of allowing for subsequent drainage and drying.

The four (4) essential design requirements for drained and back ventilated systems are:

1. Water entry through the entire wall system (penetrating the AWB) shall be prevented;

2. The AWB shall be designed to provide the primary weather protection;

3. The system shall be designed to manage and drain any water entering the cavity behind the cladding and shall be sufficiently vented to allow the cavity to dry;

4. If water vapor diffuses from the building interior through the AWB and into the drained and back ventilated system cavity, it shall be permitted to be vented and/or drained out to the exterior.

The big difference with AAMA 509 is that other than not allowing water penetration through the entire wall system, there are no other pass/fail criteria. Remember that we expect water to reach the AWB. This is a matter of risk that the specifier needs to understand. That is, the AWB becomes the primary sealing line of the envelope as the rain screen (exterior cladding element) is there to shed bulk water. The question is, “How much water contacts the AWB?” and further, when water contacts this surface, “Is the system capable of venting and drying over time?”

The classification system allows for comparison between products. The performance measurements obtained are:

• The amount (volume) of water that is collected off the AWB;

• The air flow measurements across the cladding elements.

Gutter Behind Cladding System at Sill of Drained and Back Ventilated Rain Screen Wall Cladding System (Allows for Collection of Water on Face of AWB)

To clarify, the test method incorporates a drainage gutter at the sill, as well as gutters behind the purposely designed defects/holes in the AWB to collect any water that runs down or through the AWB respectively. This water is then measured after each of four water tests; two static and two dynamic, and the average liquid ounces per square foot that contacts the AWB is calculated. Air flow is similarly determined by measuring each cladding joint and then determining the systems capacity to allow for free air flow which equates to drying potential. This data (“W” for Water and “V” for Ventilation) is then plotted on a chart and a “W-V” classification is obtained.

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